As used herein, the terms “vacuum”, “suction” and “negative pressure” have equivalent meanings and refer to an amount of air pressure developed in a confined space (e.g. piping, suction chamber, etc.) that is lower than ambient atmospheric air pressure.
As noted above, the invention is applicable to various types of filtration operation, but is particularly suitable for filtration in papermaking processes. Although the invention is described below primarily in relation to papermaking processes, it should be understood that the principles of the invention are not limited thereto, and can be applied to other filtration processes having similar requirements to papermaking processes.
In modern papermaking processes, a very dilute slurry comprising about 1% papermaking solids in about 99% water (referred to as the “stock”) is jetted at high speed and precision from a headbox onto an endless rotating belt called a forming fabric. The stock jet is aimed so that it lands on the forming fabric as the fabric passes in sliding contact over a forming element. Water from the stock drains through the forming fabric, leaving behind an embryonic mat of papermaking fibers and solids. The forming fabric and mat thereon pass over one or more drainage, agitation and suction devices which serve to both drain water and randomize the fiber distribution so as to provide a fibrous web. This web is transferred at the end of the forming section to a downstream press section where a further portion of the water is removed by mechanical means; the wet sheet is subsequently transferred to a dryer section of the papermaking machine where the remainder of the water in the web is removed by evaporative means.
Controlled sources of vacuum are used in the forming and press sections of the papermaking machine to assist in removal of water from the web and to help control agitation of the stock. Due to the highly fluid nature of the stock in the forming section, precise control of the vacuum pressure applied to the forming fabric and embryonic web through the drainage and agitation devices is very important to the properties of the finished sheet. Other factors which raise the requirement for precise control in the press section of the papermaking machine are noted below.
There are two general categories of suction assisted drainage devices presently used in papermaking processes: a) so-called low vacuum units, which supply from 0 to 60″ (0 to 1500 mm) of water vacuum to the forming fabric and web, and b) high vacuum units, which supply a higher vacuum level to the fabrics and web than low vacuum units. It has been found that valves presently in use in both of these types of suction assisted drainage devices do not provide acceptably consistent vacuum control outside the middle portion of their operating ranges.
Conventionally, vacuum control to these devices has been accomplished using a wide variety of standard and specialized valves of differing size, all of which are intended to control either low or high vacuum air flow. These valves typically provide reasonably good flow control at the middle range of their openings, but as they initially open, or as they approach their maximum opening, there is little, if any, control available. In relatively lower vacuum applications used in papermaking processes, there is a high pressure drop across the valve and a relatively low flow rate, whereas in relatively higher vacuum applications, the pressure drop across the valve is very small and the flow rates are high. In general operation, to avoid the problems noted above, it has been preferred to use a control valve that operates in the middle portion of its stroke (40 to 60% open) to achieve accurate vacuum control. This then requires a wide range of valve sizes and an accurate estimate of the operating condition of the drainage elements, which conditions may change with time and paper grade being manufactured.
The problem of reliably controlling the vacuum pressure applied to the forming fabric in particular is further complicated as a mat of papermaking fibers is built up during the sheet formation process. The amount of fiber and fillers present on a forming fabric during the papermaking process will be dependent on the paper grade being manufactured. Relatively “heavier” grades (i.e., products having relatively higher basis weights), such as linerboard or cardboard, will comprise greater amounts of fiber than comparatively “lighter” paper grades (i.e., those having comparatively lower basis weights), such as newsprint or towel, or fine papers. The heavier paper grades will form a thicker mat, which will provide greater resistance to air flow in comparison to a mat formed for a lighter grade application. This phenomenon is referred to as “filtration resistance” and is well known in the papermaking arts (see e.g. Wildfong et al. “Filtration Mechanics of Sheet Forming. Part 1: Apparatus for Determination of Constant-Pressure Filtration Resistance”. J. Pulp Paper Sci., Vol. 26, No. 7 (July 2000), pp. 250-254). As the filtration resistance of the mat increases, the amount of applied vacuum must be adjusted correspondingly to maintain adequate drainage of the sheet. Precise linear regulation of applied vacuum to the forming fabrics has been difficult to accomplish using the vacuum control valves presently available, because of this variation of the filtration resistance of the mat of papermaking fibers, based on the grade of paper product being manufactured. This problem is further exacerbated by the variety of pipe sizes used in vacuum control systems for papermaking processes. It is not uncommon to find in such systems various valves of differing sizes and designs; given these complications, adequate vacuum control during papermaking processes has been difficult to achieve.
The present invention therefore addresses the problem of valve size and design selection for use in connection with a suction assisted drainage device, and is particularly directed at the unique operating conditions and environments in which these vacuum devices are located in papermaking processes. The present invention is particularly concerned with a valve apparatus which is used to control the amount of vacuum applied to the suction assisted drainage elements so that the applied vacuum varies linearly as the valve is opened or closed in response to adjustment from a very low to a very high level of vacuum pressure. The valve is designed to provide a non-linear continuously increasing rate of change in open area which provides a linear vacuum response that is directly proportional to the valve aperture position over up to at least about 90% of its operating range. The unique configuration of the valve aperture, or open area, of the valves of the invention, allows for a linear change in the fluid flow rate (vacuum) in response to gradual linear adjustments of the valve from fully closed to fully open. This allows a single valve to be used for both high and low flow environments with more effective control of vacuum under both conditions. With a single valve for all positions in a papermaking vacuum system, changes in operating conditions do not require a change in valve size, thus significantly reducing the cost of spares. The valves of the invention provide a relatively linear change in the amount of vacuum pressure applied to the papermaking process in response to their adjustment; depending on the vacuum range being controlled, this change is directly proportional to the amount of open area provided to the vacuum source as the valve is opened or closed.
The single valves of the invention can be provided as rotary sleeve type valves, in which the flow rate is regulated by adjusting the operational area of an aperture in the rotary sleeve presented to a valve body outlet, or can be provided as other types of movable valve, for example by providing the aperture in a movable member such that the operational area presented to the valve body outlet is adjusted by moving the member in a selected direction in relation to that outlet.